AS/A-Level Chemistry - Variable oxidation states

Let's learn about transition metals having ions of different colours with their variable oxidation states in A-Level Chemistry!

Reaction of Iron

Iron (II) can be oxidised to Iron (III) in acid solution via titration.

Iron (II) is oxidised to Iron (III) when it reacts with manganate (VII) ions MnO₄^-:

Manganese is reduced from +7 to +2: 

MnO₄^- + 5e^- + 8H⁺ --> Mn²⁺ + 4H₂O

Iron is oxidised from +2 to +3:

Fe²⁺ --> Fe³⁺ + e^-

The same thing happens when iron (III) reacts with dichromate (VI) ions Cr₂O₇^2-:

Chromium is reduced from +6 to +3:

Cr₂O₇^2- + 6e^- + 14H⁺ --> 2Cr³⁺ + 7H₂O

Iron is oxidised from +2 to +3:

Fe²⁺ --> Fe³⁺ + e^-

Reaction of Chromium

In aqueous solution, Cr(VI) exists either as a chromate(VI) ion or a dichromate(VI) ion depending on the acidicty of solution. The equilibrium is established below:

The dichromate(VI) is more stable in acidic conditions and the chromate(VI) in alkaline conditions.

IN acid solutions, Cr3+and Cr2+ ions can be formed with zinc as the reducing agent:

Chromium is reduced from +6 to +3:

Cr₂O₇^2- + 6e^- + 14H⁺ --> 2Cr³⁺ + 7H₂O

Zinc is oxidised from 0 to +2: 

Zn --> Zn²⁺ + 2e^-

Then there is further reduction for chromium from 3+ to 2+:

In alkaline solution, Cr³⁺ can be oxidised to CrO₄^2- using H₂O₂ and NaOH:

Reaction of Cobalt

Cobalt (II) can be oxidised to cobalt (III) using H₂O₂ in alkaline solution:

In ammoniacal solution, Co²⁺ ions can also be oxidised to Co³⁺ ions when left in air:

Cobalt is oxidised from +2 to +3:

[Co(NH₃)₆]²⁺ --> [Co(NH₃)₆]³⁺ + e^-

Oxygen is reduced from 0 to -2:

O₂ + 2H₂O --> [Co(NH₃)₆]³⁺ +4OH^-

4 [Co(NH₃)₆]²⁺  +O₂ + 2H₂O --> [Co(NH₃)₆]³⁺ +4OH^-

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